Electrical contact



Nov. 5, 1957 T. F. EGAN 2,812,406

ELECTRICAL- CONTACT Filed March 2, 1954 FIG.

//v l/E/V TOR 7? F. E GAN ATTORNEY ELECTRICAL CONTACT Thomas F. Egan, Scotch Plains, N. 1., assignor to Belt Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application March 2, 1954, Serial No. 413,633

19 Claims. (Cl. 200-166) This invention relates generally to electrical contacts and more particularly to electrical contacts used to make and break circuits wherein circuit interruptions due to malfunctioning of contacts must be kept to a minimum.

An object of this invention is to reduce to a minimum circuit interruptions due to contact failure.

A further object of this invention is to improve circuit performance most efficiently and at the least cost, thereby resulting in the reduction of maintenance cost on contact actuating devices and their associated circuitry.

A feature of this invention is a single combinational contact which will properly make and break circuits regardless of whether or not they carry current or have a potential thereon.

A specific feature of this invention is a contact comprising an exposed layer of an alloy including by weight at least 50 percent gold and a negligible amount of metal from the group consisting of platinum, palladium, iridium, rhodium, ruthenium and osmium and an unexposed layer of an alloy thereunder including by weight at least 50 percent metal of said group or 50 percent silver or some combination thereof.

Heretofore in the prior art the solution to an electrical contact problem has always been, of necessity, a compromise. it is desirable to have a low resistive circuit path and the contacts should be mechanically strong and relatively quiet in their operation. No one metal measures up to these requirements in all respects and those" coming closest are generally the most expensive. Hence, the compromise has always been one principally between economics and performance.

The base metals, e. g., copper, nickel, etc., are generally inexpensive as well as resistant to erosion and to other deleterious effects of arcing. Even so, the formation of oxide and/ or sulfide films on their surfaces with the resulting large increase in contact resistance has always made them a poor choice for general contact use, I such as in telephone systems. however, that these films are only likely to prove objectionable when the relative movement between contacts is small.

On the other hand, the platinum-group metals which I include platinum, palladium, iridium, rhodium, ruthenium and osmium, when used as contact material will continue to provide a low resistive .circuit path as they do not suffer from the formation of oxide or sulfide films on their surfaces. The platinum group metals, however, do have the disadvantage of being relatively expensive.

Gold, electrically speaking, has been recognized in the prior art for many years as an excellent contact material. As is the case with the platinum-group metals, gold provides a dependable low resistive circuit path which is not affected by surface film formations. However, gold, as compared with the platinum group metals, is subject to excessive deterioration from mechanical wear as well as electrical erosion. Its use on contacts making or breaking appreciable amounts of current (say greater than 0.4 ,amperewhas been found generally unsatisfactory. The

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considerable expense of gold is of course an added deterrent to its use.

Silver has also been used for many years as a contact material. It, too, provides a low resistive path but has as an inherent disadvantage a propensity to tarnish when exposed to the atmosphere. This increases the resistance of a circuit path and more importantly precludes silver contacts from being used satisfactorily in many talking circuits because of the so-called tarnish noise.

The conflicting limitations both economic and performance wise which attend the utilization of various base or precious metals by themselves have led to the use of a layer of a precious metal over a cheaper base metal. In this way the actual contact surface area is composed of a precious metal and a base metal is used as a spacer or filler to reduce costs and improve fabrication techniques. Thus, contacts of palladium fabricated in this way are in use at the present time and have generally been found to be superior to gold because of palladiums superior are resistant characteristics and mechanical wear properties.

The problem of reducing or minimizing open circuits due to malfunctioning contacts on relays and other switching devices in modern telephone central oflices is a continuing one because over a million and a half contacts are variously operated, more or less continuously. It is imperative that circuit interruptions be minimized if a high standard of performance and service is to be maintained.

During intensive studies of contact opens in central offices and causes thereof, a new phenomenon has been observed in the recent past. This is the formation of a deposit on palladium contact surfaces which has the appearance of brown powder. At first, it appeared that this so-called brown powder might be dust but subsequent investigation has revealed that it is found only on platinum-group metal contacts and further, that it occurs only on those platinum-group metal contacts which open and close dry circuits. A dry circuit, as used here, designates a circuit with no voltage differential between the contacts during closure and no current flowing between the contacts during opening, i. e., a circuit producing little or no arcing when opened or closed. Investigation of Wet circuit contacts of this group (those interposed in circuits which are on opening or closing) show little or no evidence of this brown powder formation.

Further field and laboratory investigation has revealed that this brown powder is an organic deposit of good insulating properties which forms on dry circuit platinum group metal contacts when operated in the presence of organic materials. Though it is not certain by what process this organic deposit forms, there is good evidence to show that it is the result of organic vapors in the atmosphere becoming adsorbed on the surface of the platinum group metal. The friction between the mating contact surfaces (when the electromagnetic devices operate or release) combined with what appears to be a catalytic action of platinum group metals transforms the adsorbed vapors into the finely divided solid organic deposit. The collection of quantities of this organic deposit on mating surfaces causes circuit opens and occasionally produces variable-resistance contacts which create undesirable noise in telephone circuits. This organic deposit causes circuit opens in a manner similar to those caused by the presence of dust on contact surfaces; thus, contact opens that for years have been attributed to dust may well have been due, in many cases, to the presence of an organic deposit.

Additional investigation has revealed that this organic deposit does not form at all or forms to a negligible extent on gold and silver even when they are used to make and break dry circuits.

This data on brown powder formation and its possible causes, establishes one more limitation to the compromise for the most satisfactory contact material and configuration. Though gold appears to circumvent the organic deposit formation, its mechanical wear and electrical erosion properties are so poor that its use would be unsatisfactory as a general purpose contact for both wet and dry circuits.

From this dilemma came an embodiment of the present invention, a thin overlay of gold on a palladium base. With this construction, whenever a pair of contacts in a dry circuit is opened or closed the arcing, and consequently the erosion, is negligible and the circuit is satisfactorily closed between the gold overlay surfaces; whereas, when a pair of contacts in a wet circuit is opened or closed, the arcing soon erodes the thin layers of gold and the circuit is closed between the palladium contact surfaces which resist further erosion. Hence, where organic deposit formation with platinum-group metal contacts is a problem (dry circuit contacts) a circuit is closed through gold contact surfaces which do not form organic deposits. Reversely, where organic deposit formation with platinum'group metal contacts is not a problem (wet circuit contacts) the arcing wears away the thin layers of gold and allows the circuit to close through the palladium surfaces.

This solution to the circuit failure problem created by the formation of organic deposit on some platinum-group metal contacts is the most economically feasible. Another rather elaborate solution would be to code relay circuits and contact arrangements such that dry circuits functioned only through gold contacts and wet circuits functioned only through palladium contacts. This solution would be exorbitantly expensive as well as highly inflexible. Furthermore, to avoid the formation of this organic deposit by removing all or any effective quantity of organic materials from central offices and relay structures appears well-nighimpossible.

Figs. 1 and 2 of the drawing illustrate two structural configurations which this improved combinational contact may take.

In Fig. 1, a predominantly gold alloy layer 1 covers a predominantly platinum group or silver metal base layer 2 which is attached to a relay spring 3. Fig. 2 shows a predominantly gold alloy cap 4 covering a predominantly platinum group or silver metal base 5 which is, in turn, afiixed to a relay spring 6. The gold alloy overlay layer or cap may be affixed to the platinum or silver base in any convenient manner, and similarly, the base may be attached to the relay spring in any desired fashion.

A number of predominantly gold alloys can be used satisfactorily as an overlay material. While fine or pure gold will work satisfactorily from the standpoint of not encouraging the formation of organic deposits, its mechanical wear properties are so poor that it has been found advantageous to alloy it with other metals. A particular alloy including by weight 70 percent gold and percent silver has been found to work well. This alloy is disclosed in Patent 1,673,267 issued June 12, 1928 to R. A. Price, wherein it is shown as a mount wire .for a filament in an electric lamp.

An alloy of 72 percent gold, 26.2 percent silver, and 1.8 percent nickel, as disclosed in Patent 1,561,247 issued November 10, 1925 to E. F. Kingsbury, and an alloy of 72.6 percent gold, 26.4 percent silver and 1.0 percent nickel have also been found satisfactory.

Alloys comprising predominant percentages of gold with the remainder of the metal selected from a group consisting of silver, copper, nickel and other base metals are also satisfactory as overlay alloys provided the percentages of these elements alloyed with goldare kept low enough to prevent the formation of objectionable oxide or sulfide films on the surface of the gold alloy.

Also, it has been found important to keep the amount of platinum group metals alloyed with the gold at a mini mum. An alloy including by weight 69 percent gold, 25 percent silver and 6 percent platinum forms an amount of brown powder great enough to demonstrate the danger involved in allowing any appreciable amount of platinumgroup metal to be present in the gold alloy overlay. Thus, the percentage of platinum-group metal should be kept to a minimum and certainly below 5 percent.

The use of any predominantly gold alloy as described above has been found satisfactory as an overlay over a metal base including metal from the platinum group or silver. When a silver or silver alloy base is employed for contacts, however, the noise requirement of the circuit must be considered. Because long distances are spanned and intricate switching systems are employed by most American telephone installations, noise requirements are more stringent than those found in European installations. Fortunately, most talking circuits are dry circuits. Most of them close through the gold overlay and are little affected by the silver base. Wet circuits ordinarily are not talking circuits and silver-caused tarnish noise is not objectionable thereon. Quite often if the circuit requirements are not too high, the cost advantage of a silver base as compared to a platinum group metal base may outweigh the small increase in noise level which accompanies the use of a silver base.

It has been found advantageous in order to maintain a long contact life that the thickness of the 70 percent gold and 30 percent silver alloy overlay be at least .0005 of an inch thick. Preferably, the thickness of the gold alloy overlay should be between one and one and onehalf mils. With this thickness and this particular alloy, a life objective of at least a billion operations for modern telephone switching relays will not be curtailed due to the overlay alloy wearing through in dry circuits. It is,

" of course, apparent that the required thickness of the gold alloy overlay is quite dependent upon the mechanical wear properties of the particular gold alloy used and upon the mechanics of its use as well as the required life of the contacts.

It is to be understood that the above-described arrangements and alloys are illustrative of the application of the principles of the invention. Numerous other arrangements and alloy combinations may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. An electrical contact comprising an overlay cap of alloy including by weight at least 50 percent gold and not more than 5 percent metal from the group consisting of platinum, palladium, iridium, rhodium, ruthenium, and osmium, said cap covering an alloy including by weight at least 50 percent metal of said group.

2. An electrical contact comprising an exposed layer of an alloy including by weight 70 percent gold and 30 percent silver and a layer of palladium thereunder.

3. An electrical contact comprising an exposed layer of an alloy including by weight 70 percent gold and 30 percent silver and a layer of silver thereunder.

4. An electrical contact comprising an exposed layer of alloy between one and one and one-half mils thick including by weight 70 percent gold and 30 percent silver and a layer of palladium thereunder.

5. An electrical contact comprising an exposed layer of alloy at ,least one mil thick including by weight at least '50 percent gold and not more than 5 percent metal selected from the group consisting of platinum, palladium, iridium, rhodium, ruthenium, and osmium and a layer of alloy thereunder including by weight at least 50 percent metal of said group and the remainder nickel.

6. An electrical contact comprising an overlay cap of a metallic alloy at ;least one mil thick including by weight at least 50 percent gold and not more than 5 percent metal selected'from the group consisting of platinum,

palladium, iridium, rhodium, ruthenium, and osmium,

said cap covering a metallic alloy including by weight at least 50 percent silver.

7. An electrical contact comprising an exposed layer of alloy including by weight 70 percent gold and 30 percent silver and a layer of alloy thereunder. including by weight at least 50 percent metal selected from a group consisting of platinum, palladium, iridium, rhodium, ruthenium, and osmium.

8. An electrical contact comprising an exposed layer of alloy including by weight 72 percent gold, 26.2 percent silver, and 1.8 percent nickel and a layer of alloy thereunder including by Weight at least 50 percent metal selected from a group consisting of platinum, palladium, iridium, rhodium, ruthenium and osmium.

9. In combination, a pair of cooperating electrical make-and-break contacts, each of said contacts comprising an exposed layer of alloy including by weight at least 50 percent gold and not more than percent metal from the group consisting of platinum, palladium, iridium, rhodium, ruthenium, and osmium, and a layer of alloy thereunder including by weight at least 50 percent metal of said group, said exposed layers of said contacts cooperating with each other.

10. In combination, a pair of electrical contacts, each contact comprising an alloy cap at least .0005 inch thick and including by weight 70 percent gold and 30 percent silver, said cap covering an alloy including by weight at least 50 percent palladium and the remainder nickel, said caps of said contacts cooperating with each other.

11. An electrical contact comprising an exposed layer of alloy including by weight at least 50 percent gold and a negligible amount of metal from the group consisting of platinum, palladium, iridium, rhodium, ruthenium and osmium and a layer of alloy thereunder including by weight at least 50 percent metal of said group.

12. An electrical contact comprising an exposed layer of alloy including by weight at least 50 percent gold and a negligible amount of metal from the group consisting of platinum, palladium, iridium, rhodium, ruthenium and osmium and a layer of alloy thereunder including by Weight at least 50 percent silver.

13. An electrical contact comprising an exposed layer of alloy including by weight 50 to 100 percent gold, 0 to 35 percent silver, and 0 to 6 percent metal from the group consisting of platinum, palladium, iridium, rhodium, ruthenium andosmium and a layer of alloy thereunder including by weight at least 50 percent metal of said group.

14. An electrical contact comprising, in combination, an exposed layer not less than .001 inch thick of an alloy including by weight 7 2 percent gold, 26.2 percent silver,

and 1. 8 percent nickel and a layer of palladium thereunder.

15. An electrical contact comprising an exposed layer of alloy including by weight 91.7 percent gold and 8.3 percent silver and a layer of alloy thereunder including by weight at least 50 percent metal selected from the group consisting of platinum, palladium, iridium, rhodium, ruthenium and osmium.

1 6. An electrical contact comprising an exposed layer of an alloy including by weight 91.7 percent gold and 8.3 percent silver and a layer of silver thereunder.

17. In combination, a pair of cooperating electrical make-and-break contacts, each of said contacts comprising an exposed layer of alloy at least .0005 inch thick and including by weight at least 50 percent gold and a negligible amount of metal from the group consisting of platinum, palladium, iridium, rhodium, ruthenium and osmium and a layer of alloy thereunder including by weight at least 50 percent metal of said group, said exposed layers of said contacts cooperating with each other. v

18. In combination, a pair of cooperating electrical make-and-break contacts, each of said contacts comprising an exposed layer of alloy at least .0005 inch thick and including by weight at least 50 percent gold and a negligible amount of metal from the group consisting of platinum, palladium, iridium, rhodium, ruthenium and osmium and a layer of alloy thereunder including by weight at least 50 percent silver, said exposed layers of said contacts cooperating with each other.

19. In combination, a pair of cooperating electrical make-and-break contacts, each of said contacts comprising an exposed layer of alloy at least .0005 inch thick and including by weight 50 to percent gold, 0 to 35 percent silver, and 0 to 6 percent metal from the group consisting of platinum, palladium, iridium, rhodium, ruthenium and osmium and a layer of alloy thereunder including by weight at least 50 percent metal of said group, said exposed layers of said contacts cooperating with each other.

References Cited in the file of this patent UNITED STATES PATENTS 9 37,2-84 Craft et al. Oct. 19, 1909 1,561,247 Kingsbury Nov. 10, 1925 2,241,262 Keitel May 6, 1941 2,300,286 Gwyn Oct. 27, 1942 2,600,175 Volterra June 10, 1952 2,735,907 Inman Feb. 21, 1956 

11. AN ELECTRICAL CONTACT COMPRISING AN EXPOSED LAYER OF ALLOY INCLUDING BY WEIGHT AT LEAST 50 PERCENT GOLD AND A NEGLIGIBLE AMOUNT OF METAL FROM THE GROUP CONSISTING OF PLATINUM, PALLADIUM, IRIDIUM, RHODIUM, RUTHENIUM AND OSMIUM AND A LAYER OF ALLOY THEREUNDER INCLUDING BY WEIGHT AT LEAST 50 PERCENT METAL OF SAID GROUP. 